I haven't paid much attention to emacs-devel in recent months, but it doesn't look like deftheme has solved the problem of "too many mode authors setting their own colors". None of the linked themes appear to, for example, support rcirc. They support erc instead.

A good example of the color proliferation problem is egg; it defines its own diff-added and diff-removed colors, which means if you've already fixed the colors for diff-mode, you have to do the same fix again for egg.

So while, yes, color-theme didn't actually work as documented and the new stuff does, it's still practically impossible to try out various color themes.